1. Field of the Invention
This invention relates to apparatus and methods for preparing biological tissue samples for ultrastructural analysis, storage or medical use, i.e. transplantation, by avoiding significant modification of the ultrastructure of tissue during preparation of the samples themselves. It is well known in the medical arts that to examine tissue samples, and determine the cellular structure and function thereof, the tissue must be "fixed" prior to the application of nearly all analytical methodologies.
Although the phrase "tissue samples" (the term "tissue" is also used interchangeably) is used throughout this disclosure, the term should be understood to include any material composed of one or more cells, either individual or in complex with any matrix or in association with any chemical. The definition shall include any biological or organic material and any cellular subportion, product or by-product thereof. The definition of tissue samples should be understood to include without limitation sperm, eggs, embryos and blood components. The contemplated utility of the apparatus of this invention is not limited to specific types or sizes of tissue. The apparatus of this invention can be designed or adapted to any size, shape or type of cellular tissue. Therefore, the terms "tissue" and "tissue samples" are used interchangeably and are not limiting on the uses to which the method of this invention can be placed.
Although the examination of tissue by use of various microscopes or related magnifying apparatus has been practiced for many years, there has been an inherent problem in preparing tissue for use with contemporary high resolution analytical microscopes, such as the STEM electron microscopes, which permit the examination of sample constituents via X-ray analysis at powers of from 500.times. to 500,000.times. with point to point resolution of 2 to 3 Angstrom units.
Specifically, it is difficult to interpret the results of tissue analysis while concomitantly assessing the extent of various artifacts produced during the tissue preparation processes. It is thus essential that artifacts be avoided wherever possible. The term "artifact" refers to a product of artificial character due to extraneous agency. Another problem results from physical shrinkage of the tissue sample itself when subjected to the extreme procedures extant in current dogma. In most currently used tissue preparation steps, tissue shrinkage is in the order of 40% to 50%. This shrinkage inevitably results in alteration of ultrastructure and massive rearrangement of infrastructural resolution. The net result of this is ultrastructural translation damage and inaccurate detail in descriptions via existing analytical procedures.
During the so-called "Golden Age of Morphology" the predominant underlying goal in qualitative and quantitative microscopy has been an aesthetically pleasing image. This goal is readily attainable with the fixation methods and apparatus which are currently available. However, it has become essential that the aesthetically pleasing image, which is produced by the preparation process, also yield a tissue sample which accurately reflects the true condition of tissue in the living organism, i.e., approaching the "living state." This is the problem which the apparatus of this invention addresses and solves. Magnification apparatus which are currently available for analytical use are technically more advanced than are current tissue preparation techniques which have been previously employed. The method of this invention results in the preparation of tissue samples which are readily usable on known magnification and analytical apparatus.
Although the primary thrust of this application is in the preparation of tissue samples for analysis by current magnification apparatus, the invention is not intended to be so limited. More specifically, the "preparation" of tissue should be understood to refer to preparation of tissue for analysis as well as the cryofixation of tissue in anticipation of transplantation, modification, in vitro or in vivo cellular growth, fertilization, animated suspension or the more typical resin impregnation, setting, infiltration and analysis. The method of this invention can be used to prepare tissue for any medical or analytical procedure without the substantial ultrastructural damage previously thought to be inevitable in cryopreparation. For purposes of this application it should be understood that certain changes to cell morphology may occur upon application of cryoprotectants. The changes that occur are not considered substantial and are acceptable for the defined end use of the tissue.
The apparatus used in the practice of this invention is to be distinguished from contemporary freeze-drying apparatus. Freeze-drying is a technique which is well known in the art together with the equipment necessary to implement such freeze drying. See, for example, U.S. Pat. No. 4,232,453. Although in certain freeze-drying techniques liquid nitrogen is used as a cooling medium, the tissue or sample itself does not attain such temperature.
The vacuum levels disclosed and used in the apparatus used in the practice of this invention cannot be achieved safely with prior art freeze drying equipment. Typical of previous methods for drawing vacuums in freeze drying methods and apparatus is the above-mentioned U.S. Pat. No. 4,232,453 which discloses the use of molecular sieves in glass containers. Molecular sieves in easily compromised containers cannot be used safely to create and maintain the required vacuum levels to achieve the partial pressures required for sublimation of water at the anticipated temperatures (-120.degree. C. or below) created by the apparatus of the disclosed invention.
In ultra-rapid cooling apparatus and methods such as disclosed in copending commonly-assigned U.S. patent application Ser. No. 102,395 filed Sept. 29, 1987 entitled "Cryo-Slamming Apparatus and Method for Ultrarapid Cooling of Biological Samples" there is a gradient effect in the formation of ice crystals in samples more than a few microns in thickness. The ice will range from amorphous at the leading edge to hexagonal in the interior of the tissue. Accordingly, while that apparatus and method have proven to be extremely valuable for small samples, their usefulness is limited by the size of the samples.
2. The Prior Art
The most common prior art method for preparation of tissue samples for analysis is by means of chemical fixation and organic solvent dehydration. Inherent in prior art processes is the concomitant artifact creation, sample shrinkage and resultant damage to and modification of tissue characteristics. These tissue characteristic modifications, whether in the form of artifacts or the like, require interpretation by the individual or apparatus analyzing or evaluating the sample. This introduces, in many instances, an unsatisfactory risk of error.
Chemical fixation is a well known technique and has served the analytical biologist well for many years and undoubtedly will continue to do so in certain limited applications. However, as the use of tissue sample analysis becomes more complex and the use of such analysis becomes more widespread, alternatives to chemical fixation are demanded. This is especially true as advances are being made in the magnification and analytical resolution of apparatus which are available. It is necessary that tissue preparation methods and the apparatus necessary to prepare tissue samples be equally advanced as the analytical tools, i.e., electron microscopes, which are being used to analyze the samples. Obviously, if the technology for tissue sample preparation is behind the technology of microscopy then the advanced microscopes cannot be used to full advantage.
Similarly, it is essential that cryopreparation methods and apparatus develop concurrently with other medical technology, i.e., surgical transplant techniques, bio-engineering and biogenetics. In short, cryopreparation is an essential intermediate step in evolving processes using or analyzing cells or tissue. If cryopreparation apparatus does not evolve then the thrust of medical technology into unexplained and unexplored medical arts will be blunted. The method of this invention represents a cryopreparation breakthrough that will permit research into the use and preparation of biological tissue to keep pace with other advances in medical technology.
The most common alternative to chemical fixation and organic solvent dehydration is freeze drying cryofixed samples. Freeze-drying following cryofixation is a well documented and well known technique for tissue preservation. It has several advantages. Cryofixation results in a near-instantaneous arrest of cellular metabolism. Freeze drying results in a stabilization and retention of soluble cell constituents through elimination of solvent contact with the sample. These are significant advantages to cryofixation freeze-drying that have resulted in a great deal of research in attempting to apply cryofixation and freeze-drying techniques to known tissue preparation processes.
Unfortunately, freeze-drying technology inherently possesses a number of disadvantages relevant to tissue preparation methodologies. The primary disadvantage in currently available freeze-drying techniques and apparatus is the inherent formation of ice crystals. As can be readily appreciated, the formation of ice crystals destroys the ultrastructural integrity of the tissue sample being reviewed. The image is distorted and the cytoplasm becomes reticulated. The formation of ice crystals in the sample can also result in a change in pH within microcompartments of the tissue (eutectic formation) which possibly can result in abnormal tertiary conformation of macromolecules. There is also the possibility that proteins will denature and precipitate. These are but a few of the disadvantages which are inherent in the freeze-drying process.
This general topic is discussed in some detail together with other prior art methods in an article entitled Freezing and Drying of Biological Tissues for Electron Microscopy, Louis Terracio and Karl G. Schwabe, published in The Journal of Histochemistry and Cytochemistry, Volume 29, No. 9 at pp. 1021-1028 (1981). Problems associated with artifact formation are described in Understanding the Artefact Problem in Freeze-Fracture Replication: A Review, The Royal Microscopial Society, (1982) at pp. 103-123.
A general principle found applicable to cryofixation techniques, which has demonstrated utility in the preparation of tissue samples, is that as the cooling rate increases, tissue fluids can be vitrified. It has been postulated that regardless of the rate of cooling, ice crystals may still be formed, but as the cooling rates increase the size of the intracellular ice crystals decreases. The small size or absence of ice crystals at high freeze rates is of course a substantial advantage in morphology retention as this results in minimal artifact creation and minimal ultrastructural damage during tissue dehydration. The method of this invention requires the cooling of tissue samples to the vitreous phase followed by dehydration of the tissue sample while in the state of reduced partial pressure of water vapor, all without substantial ultrastructural damage to the tissue cells.
For purposes of this application, the term "vitreous" or "vitrification" or "vitreous phase" should be understood to refer to the physical condition of tissue upon cooling at a rate and under conditions in which resolvable ice crystals are not present and/or are not being nucleated at a rate which will result in the formation of resolvable ice crystals.
Historically, the criteria by which the techniques for supercooling have been judged was not the cooling rate of the system but simply the temperature of the environment in which the tissue was frozen. Thus, the term rapid supercooling has been applied to any system in which the supercooling agent has a temperature of -150.degree. C. or below. The effectiveness of a cooling system is dependent upon the rate at which heat is removed from the sample. Heat transfer is dependent not only on the temperature of the freezing system but also on its physical and thermal characteristics, as well as the size, pretreatment and thermal characteristics of the tissue.
The most commonly used technique for rapid supercooling is to immerse or "quench" the sample in a fluid cooling bath. The most commonly used fluids for quenching are liquid nitrogen, isopentane, propane and fluorocarbons such as Freon 12 and Freon 22. Although liquid nitrogen is generally regarded as an ideal quenching fluid due to its low temperature (-196.degree. C.), there are inherent disadvantage in the use of liquid nitrogen due to the occurrence of tissue surface film boiling caused at least in part by the low heat of vaporization of liquid nitrogen. Film boiling is a characteristic of liquid nitrogen that inhibits the heat transfer rates by actually insulating the sample.
An alternate prior method for rapid supercooling is freezing on the polished surface of a chilled metal block. This typically involves opposing the tissue sample to a polished flat metal surface by pressing it firmly against the surface of the metal. Silver and copper are typically used as the polished metal blocks. This method is designed to take advantage of the high thermal conductivities and heat capacities of these metals when cooled to liquid nitrogen or liquid helium temperatures. The critical step in chilling on the surface of a metal is making firm contact with the dry, chilled metal surface with no rotational, translational or rebounding motion. Certain commercially available apparatus having known utility in the medical arts address and provide "bounce-free" freezing. Credit for the development of this apparatus is generally accorded to Dr. Alan Boyne of the University of Maryland School of Medicine.
There has recently been verification that there is a direct correlation between cooling rate and ultrastructural preservation of pure samples in quenching fluids. As the cooling rate increases over the range of 100.degree. C. to 4100.degree. C. per second (liquid nitrogen-propane), there is a corresponding decrease in the size of ice crystals formed and thus an improvement in morphological preservation.
The critical steps in the subsequent tissue preparation process are invariably stimulated sublimation-dehydration of the supercooled tissue fluids, which have recently been described as a stimulated "molecular distillation" process. Once the appropriate supercooling method has been chosen and implemented, it is sometimes necessary to further process the tissue for microscopic evaluation, since electron microscopes or other magnification apparatus that allow the viewing of frozen hydrated specimens are not readily available. Thus, dehydration is an essential step in the preparation of biological tissue samples for storage and a step which oftentimes results in the destruction via reticulation of the infrastructure and ultrastructure of the tissue. Tissue cell destruction from dehydration not only impairs analysis by magnification apparatus but also adversely affects the functional characteristics and viability of tissue masses being used, i.e. transplanted.
In certain prior drying techniques, the tissue sample had not been entirely solidified due to eutectic formation as the cellular fluid solutes were concentrated in bound water compartments. This transfer of solute occurs while the materials are in the fluid state when slow cooling is employed. When rapid cooling techniques are used, unique procedures, which are distinct from those characteristic of freeze-drying, must be employed in the dehydration step. Problems result from the fact that dehydration must take place (the water must be removed) in the solid rather than the liquid state, i.e., via sublimation.
In the prior art, the freeze substitution approach has involved the removal of tissue water by substituting a solvent or solvent-fixative mixture for the solid phase water at -50.degree. to -80.degree. C. This introduces less severe solvent phase separation and chemical alteration artifacts to a tissue sample than past routine chemical fixation methodologies. From a practical standpoint freeze-drying is complicated by the requirement that the tissue sample be warmed to increase the vapor pressure of the supercooled water and allow sublimation to proceed in a reasonable period of time. The increased temperature, in addition to increasing vapor pressure can produce a series of physical events leading to the expansion of ice crystals and concomitant damage to the ultrastructural morphology of the tissue sample. Many of the physical events which occur during the warming process have to do with transitions in the physical state of the water which is present. Changes which are typically encountered are glass transition, devitrification and crystallization with an ensuing series of crystal lattice configurations.
An alternate procedure which has been used successfully is stimulated molecular distillation. Stimulated molecular distillation refers to a process in which the amount of energy in the antibonding orbitals of surface molecules is elevated, enabling them to escape to the gas phase and not be recaptured by the solid phase.
Thus it can be appreciated that freeze-drying technology and cryopreparation techniques present an exceptional opportunity for the preparation of tissue samples for morphological examination. However, inherent in the use of freeze-drying techniques are problems associated with dehydration and fixation of samples. Supercooling processes followed by molecular distillation have resulted in major improvements over freeze-drying techniques. However, these processes require fairly long drying times and the thickness of the vitreous phase of water is limited. These are some of the problems which are addressed by the process and apparatus of this invention.
One advantage of this invention is the ability to cryoprepare tissue without overt disruption or destruction of the morphological characteristics of the ultrastructure of tissue cells. This invention permits the cryopreparation of tissue by dehydrating tissue maintained in the solid, vitreous phase without creating unnecessary artifacts which restrict interpretation by conventional analytical apparatus and renders these suitable for transplantation.